Mail is stuck in queue, postfix -f doesn't do anything.
However, if I reboot the first 30 queued emails are sent then the rest gets stuck again.
I have diverted outgoing mail to a different route so the queue is not growing.
But with 2000 emails stuck I'd need to reboot another 67 times to get them flushed out.

edit: I fixed it myself.
I deleted everything from the queue with 'MAILER DAEMON' in it, which were mostly bounces to out office messages.